Through air drying has become the technology of preference for making tissue for many manufacturers who build new tissue machines as, on balance, through air drying (xe2x80x9cTADxe2x80x9d) offers many economic benefits as compared to the older technique of conventional wet-pressing (xe2x80x9cCWPxe2x80x9d). With through air drying, it is possible to produce a single ply tissue with good initial softness and bulk as it leaves the tissue machine.
In the older wet pressing method, to produce a premium quality tissue, it has normally been preferred to combine two plies by embossing them together. In this way, the rougher air-side surfaces of each ply may be joined to each other and thereby concealed within the sheet. However, producing two-ply products, even on state of the art CWP machines, lowers paper machine productivity by about 20% as compared to a one-ply product In addition, there may be a substantial cost penalty involved in the production of two-ply products because the parent rolls of each ply are not always of the same length, and a break in either of the single plies forces the operation to be shut down until it can be remedied. Also, it is not normally economic to convert older CWP tissue machines to TAD. But even though through air drying has often been preferred for new machines, conventional wet pressing is not without its advantages as well. Water may normally be removed from a cellulosic web at lower energy cost by mechanical means such as by overall compaction than by drying using hot air.
What has been needed in the art is a method of making a premium quality single ply tissue using conventional wet pressing having a high bulk and excellent softness attributes. In this way advantages of each technology could be combined so older CWP machines can be used to produce high quality single ply tissue at a cost which is far lower than that associated with producing two-ply tissue.
Among the more significant barriers to the production of a single ply CWP tissue have been the generally low softness, thinness and the extreme sidedness of single ply webs. A tissue product""s softness can be increased by lowering its strength, as it is known that softness and strength are inversely related. However, a product having very low strength will present difficulties in manufacturing and will be rejected by consumers as it will not hold up in use. Use of premium, low coarseness fibers, such as eucalyptus, and stratification of the furnish so that the premium softness fibers are on the outer layers of the tissue is another way of addressing the low softness of CWP products; however this solution is expensive to apply, both in terms of equipment and ongoing fiber costs. In any case, neither of these schemes addresses the problem of thinness of the web. TAD processes employing fiber stratification can produce a nice, solf, bulky sheet having adequate strength and good similarity of the surface texture on the front of the sheet as compared to the back. Having the same texture on front and back is considered to be quite desirable in these products or, more precisely, having differing texture is generally considered quite undesirable. Because of the deficiencies mentioned above, many single-ply CWP products currently found in the marketplace are typically low end products. These products often are considered deficient in thickness, softness, and exhibit excessive two sidedness. Accordingly, these products have had rather low consumer acceptance and are typically used in xe2x80x9caway from homexe2x80x9d applications in which the person buying the tissue is not the user.
We have found that we can produce a soft, high basis weight, high strength CWP tissue with low sidedness having a serpentine configuration by judicious combination of several techniques as described herein. Basically, these techniques fall into four categories: (i) providing a web having a basis weight of at least 15 pounds for each 3000 square foot ream; (ii) adding to the web a controlled amount of a temporary wet strength agent and softener/debonder; (iii) low angle, high percent crepe, high adhesion creping giving the product low stiffness and a high stretch; and (iv) optionally embossing the tissue. By various combinations of these techniques as described, taught, and exemplified herein, it is possible to almost xe2x80x9cdial inxe2x80x9d the required degree of softness, strength, and sidedness depending upon the desired goals. The use of softeners having a melting range of about 1xc2x0-40xc2x0 C. and being dispensable at a temperature of about 1xc2x0-100xc2x0 C. suitably 1xc2x0-40xc2x0 C. preferably 20xc2x0-25xc2x0 C. further improves the properties of the novel one-ply tissue having a serpentine configuration.
The present invention is directed to a soft, strong in use, bulky single ply tissue paper having a serpentine configuration and a low sidedness and processes for the manufacture of such tissue.
Paper is generally manufactured by suspending cellulosic fiber of appropriate geometric dimensions in an aqueous medium and then removing most of the liquid. The paper derives some of its structural integrity from the mechanical arrangement of the cellulosic fibers in the web, but most by far of the paper""s strength is derived from hydrogen bonding which links the cellulosic fibers to one another. With paper intended for use as bathroom tissue, the degree of strength imparted by this inter-fiber bonding, while necessary to the utility of the product, can result in a lack of perceived softness that is inimical to consumer acceptance. One common method of increasing the perceived softness of bathroom tissue is to crepe the paper. Creping is generally effected by fixing the cellulosic web to a Yankee drum thermal drying means with an adhesive/release agent combination and then scraping the web off the Yankee by means of a creping blade. Creping, by breaking a significant number of inter-fiber bonds adds to and increases the perceived softness of resulting bathroom tissue product.
Another method of increasing a web""s softness is through the addition of chemical softening and debonding agents. Compounds such as quaternary amines that function as debonding agents are often incorporated into the paper web. These cationic quaternary amines can be added to the initial fibrous slurry from which the paper web is subsequently made. Alternatively, the chemical debonding agent may be sprayed onto the cellulosic web after it is formed but before it is dried.
One-ply bathroom tissue generally suffers from the problem of thinness, lack of softness, and also xe2x80x9csidedness.xe2x80x9d Sidedness is introduced into the sheet during the manufacturing process. The side of the sheet that was adhered to the Yankee and creped off, i.e., the Yankee side, is generally softer than the xe2x80x9cairxe2x80x9d side of the sheet This two-sidedness is seen both in sheets that have been pressed to remove water and in unpressed sheets that have been subjected to vacuum and hot air (through-drying) prior to being adhered to the crepe dryer. The sidedness is present even after treatment with a softener. A premium one-ply tissue should not only have a high overall softness level, but should also exhibit softness of each side approaching the softness of the other.
The most pertinent prior art patents will be discussed but, in our view, none of them can be fairly said to apply to a one-ply tissue of this invention which exhibits high thickness, soft, strong and low sidedness attributes. U.S. Pat. No. 4,447,294, issued to Osbom, III, relates to towels and facial tissue and discloses a process for making a towel or facial tissue product having high wet strength and low dry strength. This reference requires that the wet strength agent be at least partially cured and that a debonding agent be applied to the already dried web, which further distinguishes that reference from the present invention. Phan et al., in U.S. Pat. No. 5,262,007 discloses towels, napkins, and tissue papers containing a biodegradable softening compound, a temporary wet strength resin, and a wetting agent The Phan reference requires the use of a wetting agent, presumably to restore the absorbency lost by use of the softening agent. The present invention is unrelated to the Phan reference and does not require use of a wetting agent to achieve a one-ply bathroom tissue having high absorbency. In U.S. Pat. No. 5,164,045, Awofeso et al. disclose a soft, high bulk tissue. However, production of this product requires stratified foam forming and a furnish that contains a substantial amount of anfractous and mechanical bulking fibers, none of which are necessary to practice the present invention.
European Application 95302013.8 discloses a low sidedness product, but the tissue does not have the high thickness and temporary strength agent of the present invention. In addition, production of this product requires such strategies as fiber and/or chemical stratification that have been found unnecessary to produce the product of the present invention. Dunning et al., U.S. Pat. No. 4,166,001, discloses a double creped three-layered product having a weak middle layer. The Dunning product does not suggest the one-ply premium softness soft tissue of this invention having a serpentine configuration and does not contain a temporary wet strength agent.
The foregoing prior art references do not disclose or suggest a high-softness, strong one-ply tissue having serpentine configuration and low sidedness and having a total specific tensile strength of no more than 200 grams per three inches per pound per 3000 square foot ream, a cross direction wet tensile strength of at least 2.75 grams per three inches per pound per 3000 square foot ream, a specific geometric mean tensile stiffness of 0.5 to 3.2 grams per inch per percent strain per pound per 3,000 square foot ream, a GM friction deviation of no more than 0.25 and a sidedness parameter less than 0.3. The sidedness parameter goes down when the total specific tensile strength is reduced. Thus at a total specific tensile strength of no more than 75 grams per three inches per pound per 3000 square foot, the sidedness parameter is usually in the range of about 0.18 to 0.25.
The novel premium quality high-softness, single-ply tissue having a serpentine configuration and a very low xe2x80x9csidednessxe2x80x9d along with excellent softness, coupled with strength is advantageously obtained by using a combination of four processing steps.
Suitably, the premium softness, strong, low sidedness bathroom tissue has been prepared by utilizing techniques falling into four categories: (i) providing a web having basis weight of at least 15 pounds for each 3000 square foot ream; (ii) adding to the web or to the furnish controlled amounts of a temporary wet strength agent and a softener/debonder preferably a softener dispersible in water at a temperature of about 1xc2x0-100xc2x0 C. suitably 1xc2x0-40xc2x0 C. advantageously 20xc2x0-25xc2x0 C. Advantageously the softener should have a melting point below 40xc2x0 C.; (iii) low angle, high adhesion creping using suitable high strength nitrogen containing organic adhesives and a crepe angle of less than 85 degrees, the relative speeds of the Yankee dryer and reel being controlled to produce a product having a final product MD stretch of at least 15%; and (iv) optionally embossing the tissue. The furnish may include a mixture of softwood, hardwood, and recycled fiber. The premium softness and strong single-ply tissue having low sidedness may be suitably obtained from a homogenous former or from two-layer, three-layer, or multi-layer stratified formers.
Further advantages of the invention will be set forth in part in the description which follows. The advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.
To achieve the foregoing advantages and in accordance with the purpose of the invention as embodied and broadly described herein, there is disclosed:
A method of making a high-softness, high-basis weight, single-ply tissue having a serpentine configuration comprising:
(a) providing a fibrous pulp of papermaking fibers;
(b) forming a nascent web from said pulp, wherein said web has a basis weight of at least about 15 lbs./3000 sq. ft ream;
(c) optimally including in said web at least about 3 lbs./ton of a temporary wet strength agent and up to 10 lbs./ton of a nitrogen containing softener; optionally a cationic nitrogen containing softener; dispersible in water at a temperature of about 1xc2x0-100xc2x0 C. suitably 1xc2x0-40xc2x0 C. advantageously 20xc2x0-25xc2x0 C., advantageously the softener has a melting point below 40xc2x0 C.;
(d) dewatering said web;
(e) adhering said web to a Yankee dryer,
(f) creping said web from said Yankee dryer using a creping angle of less than 85 degrees, wherein the relative speeds between said Yankee dryer and the take-up reel is controlled to produce a final product MD stretch of at least about 15%;
(g) optionally calendering said web;
(h) optionally embossing said web; and
(i) forming a single-ply web wherein steps (a)-(f) and optionally steps (g) and (h) are controlled to result in a single-ply tissue product having a serpentine configuration and a total specific tensile strength of no more than 200 grams per three inches per pound per 3,000 square foot ream, suitably no more than 150 grams per three inches per pound per ream, preferably no more than 75 grams per three inches per pound per 3000 square foot ream, a cross direction wet tensile strength of at least 2.7 grams per three inches per pound per ream, a specific geometric ream tensile stiffness of between 0.5 and 3.2 grams per inch per percent strain per pound per 3000 square foot ream, a GM friction deviation of no more than 0.25 and a sidedness parameter less than 0.3 usually in the range of about 0.180 to about 0.250 when the total specific tensile strength does not exceed 75 grams per three inches per pound per 3000 ream.
To summarize at a total specific tensile strength of about 200 grams per 3 inches or less per 3000 square foot ream, the cross direction specific wet tensile strength is about 20 grams or less per 3000 square foot ream, the ratio of MD tensile to CD tensile is between 1.25 and 2.75. The specific geometric mean tensile strength is 3.2 or less grams per inch per percent strain per pound per 3000 square foot ream. The friction deviation is less than 0.25 and the sidedness parameter is less than 0.30. At a total specific tensile strength of about 150 grams per 3 inches or less per 3000 square foot ream the cross direction specific wet tensile strength is about 15 grams or less per 3000 square foot ream, the ratio of MD tensile to CD tensile is between 1.25 and 2.75. The specific geometric ream tensile strength is 2.4 or less grams per inch per percent strain per pound per 3000 square foot ream. The friction deviation is less than 0.25 and the sidedness parameter is less than 0.30. When the tissue product exhibits a total specific tensile strength between 40 and 75 grams per 3 inches per 3000 square foot ream, it has a cross direction specific wet tensile strength of between 2.75 and 7.5 grams per 3 inches per pound per 3000 square foot ream, and its specific geometric mean tensile stiffness is between 0.5 and 1.2 grams per inch per percent strain per pound per 3000 square foot ream and its friction deviation is less than 0.225; and the tissue has sidedness parameter of less than 0.275.
In one embodiment of this invention, the product may be embossed with a pattern that includes a first set of bosses which resemble stitches, hereinafter referred to as stitch-shaped bosses, and at least one second set of bosses which are referred to as signature bosses. Signature bosses may be made up of any emboss design and are often a design which is related by consumer perception to the particular manufacturer of the tissue.
In another aspect of the present invention, a paper product is embossed with a wavy lattice structure which forms polygonal cells. These polygonal cells may be diamonds, hexagons, octagons, or other readily recognizable shapes. In one preferred embodiment of the present invention, each cell is filled with a signature boss pattern. More preferably, the cells are alternatively filled with at least two different signature emboss patterns.
In another preferred embodiment, one of the signature emboss patterns is made up of concentrically arranged elements. These elements can include like elements for example, a large circle around a smaller circle, or differing elements, for example a larger circle around a smaller heart. In a most preferred embodiment of the present invention, at least one of the signature emboss patterns are concentrically arranged hearts as can be seen in FIG. 7. Again, in a most preferred embodiment, another signature emboss element is a flower.
The one-ply tissue of this invention has higher softness and strength parameters than prior art one-ply tissues and the embossed one-ply tissue product of the present invention has superior attributes than prior art one-ply embossed tissue products. The use of concentrically arranged emboss elements in one of the signature emboss patterns adds to the puffiness effects realized in the appearance of the paper product tissue. The puffiness associated with this arrangement is the result not only of appearance but also of an actual raising of the tissue upward.